Heretofore a common method of fabricating silicon solar cells has included the steps of: (1) diffusing phosphorus into P-type silicon ribbon so as to form a shallow junction, (2) forming on the junction side of the ribbon a thin coating of silicon nitride, (3) forming a grid electrode pattern in the silicon nitride coating by photolithography (e.g., using a suitable photoresist composition and etching), (4) coating the other side of the silicon ribbon with an aluminum paste, (5) heating the silicon so as to alloy the aluminum and silicon, (6) plating exposed silicon on the junction side and aluminum on the other side with nickel, (7) sintering the nickel to form a nickel silicide on the junction side, and (8) plating additional metal(s) onto the metal-covered portions of the silicon. An example of such a process can be found in U.S. Pat. No. 4,451,969.
In a typical diffusion-type junction-forming operation, diffusion occurs on all of the surfaces of the substrate. Thus, for example, when phosphorus is diffused into the P-type silicon ribbon from phosphosilicate glass to form a shallow junction, the junction and phosphosilicate glass will extend along the front and back sides of the cell as well as on the edges. The glass is removed by immersing the substrate in an etching solution Nevertheless, a continuous junction will exist around the cell (as viewed in cross-section) which will provide a low resistance current path between the front and rear sides of the solar cell. In some cases, current paths between the front and rear sides of the solar cell may also be created from metallic wrap-around of metal deposited on the substrate during the formation of an electrode.
In the past, the cell edges have been trimmed to conductively isolate the back of the cell from the front. Trimming has been done by mechanically sawing off edge portions of the cells. The saw cutting technique for trimming the edges is effective in isolating the front and back surfaces, but it is also wasteful. The trimmed cells are smaller and the material trimmed is generally discarded as waste. Unfortunately, safe disposal of silicon waste is both difficult and costly. It is costly not only in terms of material waste, but also in that the silicon must be disposed of within the guidelines established by the government pollution control authorities. Cell edges also have been trimmed by plasma-etching. The latter technique also has disadvantages.
It is recognized by persons skilled in the art that widespread use of photovoltaic solar cells is dependent upon the development of fabrication techniques capable of producing reliable solar cells with a conversion efficiency of 12% or higher at a relatively low cost. The cost and saleability of solar cells, like other semiconductor devices, depends on (1) the cost of the starting materials, (2) the cost of converting the starting materials into the finished product, (3) the cost of disposing of waste materials, (4) the overall output of the cells, and (5) the yield of acceptable solar cells.